The present invention relates to a fuel pump and to a method for controlling a fuel pump.
One way of reducing discharges of emissions from diesel engines is to inject the fuel at a very high pressure. A so-called “common rail” system is commonly used for effecting injection at a high pressure in the combustion spaces of a diesel engine. A common rail system comprises a high-pressure pump which pumps fuel at a high pressure to an accumulator tank (“common rail”). The fuel in the accumulator tank is intended to be distributed to all the cylinders of the combustion engine. Fuel from the accumulator tank is injected into the combustion spaces of the respective cylinders by electronically controlled injection means.
Conventional high-pressure pumps which deliver fuel to an accumulator tank usually comprise at least two pump means. The pump means work alternately so that when one pump means pressurises fuel, fuel is fed into the second pump means. The high-pressure pump can thus deliver a substantially continuous fuel flow at a high pressure to an accumulator tank. The fuel flow to the respective pump means is controlled by a regulating valve which has a variable constriction. Depending on the load on the combustion engine, the constriction of the regulating valve is varied so that the high-pressure pump delivers a desired amount of fuel to the accumulator tank. Since the pump means work at a relatively high frequency, it is not possible to reset the constriction of the regulating valve each time when fuel is supplied to the respective pump means. The regulating valve therefore delivers a substantially equal fuel flow to each pump means. In situations where the load on the combustion engine is low, a reduced amount of fuel is supplied to the respective pump means. Thus only a small proportion of the stroke length of the pistons of the respective pump means is usable for compressing the fuel. Consequently the fuel pump will work at reduced efficiency when the load upon it is low. Moreover, the pump means will emit more noise when the load upon them is low. The reason for the increased noise emissions is that the pump means are not totally full of fuel during the compression stroke of the pistons.